Holograms are those in which the wave front of an object light beam is recorded as interference fringes in a photosensitive material by the interference between two beams (object light beam and reference light beam) having the same wavelength. When a light beam having the same conditions as a reference light beam used at the time of recording interference fringes is applied, a diffraction phenomenon by the interference fringes is caused, and whereby the same wave front as that of the original object light beam can be regenerated. Such a hologram can be divided into some types (surface relief type hologram and a volume type hologram) by the recording form of interference fringes.
Here, the surface relief type hologram is a type in which fine convexo-concave pattern is engraved on the surface of a hologram layer to record a hologram. The volume type hologram, on the other hand, is a type in which interference fringes produced by the interference of light are depicted three-dimensionally in the direction of the thickness as fringes differing in refractive index to record a hologram.
The volume type hologram can be mass-produced by using the hologram original master, and therefore has an advantage of being better in technical production compare to the relief type hologram. However, reality is that the laser beam used for technical production is limited in their wavelengths. Therefore, the wavelengths of the light which regenerate the hologram image of the volume type hologram mass-produced are also limited and it has been difficult to regenerate bright hologram images in embodiments commonly using the holograms.
To respond to such problem, methods of changing the regenerated wavelengths of the hologram images to the wavelengths different from the wavelengths of lights used at the time of recording the interference fringes are recently used. In such method, an after-treatment to the volume hologram layer where the interference fringes are recorded is carried out, and thereby the period of interference fringes initially recorded is changed.
In other words, since the regenerated wavelengths of the volume hologram are the same to the period of interference fringes recorded on the hologram layer, by changing the period of interference fringes recorded to the hologram layer afterwards, it is possible to change the period of interference fringes identical to the wavelengths of lights which are frequently used in daily bases. Such methods to change the period of interference fringes recorded to the hologram layer afterwards is useful in that it is possible to produce the volume hologram layers which can regenerate bright hologram images in common embodiments.
As such methods to change the period of interference fringes recorded to the hologram layer afterwards, various methods are known. As a more common method, a method disclosed in the Patent Document 1 is introduced here as an example. The Patent Document 1 discloses a method to enlarge the period of interference fringe by moving a monomer and/or a plasticizer contained in a layer to a volume hologram layer by carrying out a treatment such as a heating treatment through contacting the layer containing the monomer and/or the plasticizer to the volume hologram layer where the interference fringes are recorded. Such a method is certainly effective in enlarging the period of interference fringes and to shift the regenerated wavelength to the long-wavelength side. However, on the other hand, it has problems such as offering few variable amounts in the period of interference fringe or making the process complicating.
Further, since the volume holograms can record information to the thickness direction, is means to record/regenerate the three-dimension images, and is expressed by light interference colors, it has an appearance not easily obtained by other image-forming means. The producing methods of the volume hologram are known, but copying of the volume hologram is difficult because producing thereof requires a sophisticated work using optical devices. Using such characteristics of the volume holograms, the volume holograms are used for prevention of copying the identification cards, bank card and others. The present inventors have been discussing various methods as shown in Patent Documents 2 and 3 to prevent forgery by making the peeling of volume hologram layer from the adhered identification card impossible so that peeling of which results in the breakage of the attached volume hologram. However, there is a possibility of allowing a copy of the volume hologram when a contact copy by a single wavelength laser using the volume hologram laminate with a volume hologram recorded as an original master is tried. Thus, a development of a volume hologram laminate which makes a copy of the volume hologram difficult has been called for.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 3-46687    Patent Document 2: JP-A No. 63-284586    Patent Document 3: JP-A No. 2002-358018